This invention generally relates to methods and systems for high speed laser processing (machining, cutting, ablating) microstructures. More specifically, this invention relates to methods and systems for precisely positioning a waist of a material-processing laser beam to process microstructures within a laser-processing site. Semiconductor memory repair is a specific application where precise positioning in depth of the beam waist of the laser beam is required to dynamically compensate for local variations in height of the wafer or target surface.
Memory Repair is a process used in the manufacture of memory integrated circuits (DRAM or SRAM) to improve the manufacturing yield. Memory chips are manufactured with extra rows and columns of memory cells. During testing of the memory chips (while still in the wafer form), any defects found are noted in a database. Wafers that have defective die can be repaired by severing links with a pulsed laser. Systems generally utilize wafer-handling equipment that transports semiconductor wafers to the laser process machine, and obtain the information in the form of an associated database specifying where the links should be cut and performs the requisite link ablation for each wafer.
Successive generations of DRAM exploit finer device geometry in order to pack more memory into smaller die. This manufacture of smaller devices affects the geometry of the links allocated for laser redundancy. As the devices get smaller, the links get smaller and the pitch (link-to-link spacing) shrinks as well. Smaller link geometry requires a smaller spot size from the laser in order to successfully remove selected links without affecting adjacent links, preferably with little if any compromise in throughput.
All systems focus the laser-processing beam to perform memory repair and require that the surface of the link be maintained within a small tolerance of the beam waist (focus) position with depth. When the link is in the focal plane of the lens, the focused spot will be minimum size. At focus or xe2x80x9cbeam waist heightxe2x80x9d above or below nominal, the spot will be defocused with the magnitude of defocus increasing with distance from nominal. A defocused spot reduces the energy that is delivered to the target link possibly leading to insufficient cutting of the link. A defocused spot may also place more laser energy on adjacent links or on the intervening substrate leading to possible substrate damage. At some level of defocus, the laser cutting process is no longer viable.
The allowable tolerance for relative placement of the lens and link is referred to as xe2x80x9cdepth of focusxe2x80x9d (DOF). The depth of focus criteria is a function of the process tolerance for the particular link and laser combination. Experiments are typically performed over a range of operating parameters, including focus height, in order to determine the sensitivity of the laser cutting process to the parameters. For instance, from these experiments it might be found that the laser would reliably sever links when the combinations may exhibit more or less process latitude to focus height.
Prior generation memory repair systems perform a focus operation once per site. As more dies are processed within a single site, the site dimensions get larger. This presents a problem in that the wafers seldom are flat (planar) and parallel to the focal plane. If focus is performed at only one point within a site, then the system will operate slightly out of focus at points within the site that are not near to the focus location.
At least three factors affect the ability of a memory repair system to maintain the link in focus.
1. The process or sensor used to measure focus may exhibit errors.
2. The wafer may exhibit xe2x80x9ctopologyxe2x80x9d that requires different focus heights at different locations over the surface of the wafer.
3. The mechanism used to provide relative motion between the wafer surface and focal plane may exhibit errors.
A process for compensating height variations was used in 1992 by a predecessor company of the assignee of the present invention (i.e. xe2x80x9cGSIxe2x80x9d) to perform thin-film trimming on integrated circuits (IC) in non-wafer form. At the time, IC""s were being packaged into sensors and then trimmed after packaging. The problem encountered at the time was due to the packaged die being significantly non-parallel to the surrounding package (typically pressure sensors). Incorporating a Z-Roll-Pitch mechanism for positioning the device in the product solved the problem at the time. An auto-collimator sensor was included in the optical path and used to measure the angle of the die surface relative to the focal plane. The angular information from the auto-collimator was combined with a single focus measurement to define a plane. The mechanism then moved the die in 3 axes to place the die into the best-fit plane compensating for Z, roll and pitch. The range of die tilting was sufficiently large that it was often necessary to perform iterative corrections to properly focus the die. After making an adjustment in Z, roll and pitch, a second set of focus and tilt measurements was made followed by a subsequent (smaller) focus and tilt correction.
One problem of this approach is that the auto-collimator worked best when it could be directed at a large xe2x80x9cplanarxe2x80x9d object. With pressure sensors, it was often possible to define a large region that lacked surface features in order to use as an auto-collimator target. It would not be possible to find such a region on a typical IC found in memory repair applications.
In 1994, GSI developed a different approach to handle thin-film trimming on xe2x80x9ctilted die.xe2x80x9d The problem was again due to trimming on packaged IC (pressure sensors). In this case, the specifics of the customer""s device precluded the use of a tilting Z-stage. A single Z-axis stage was used in the product and the Z-stage was moved in coordination with X and Y positioning of the laser beam. Also, the absence of suitable target structures for the auto-collimator on certain customer""s devices forced GSI to develop the multi-site focus algorithm. Height measurements were obtained using a sensor that obtained a sequence of measurements along the z-axis from which the position of best focus was correlated to surface positionxe2x80x94a prior art method known as xe2x80x9cdepth from focusxe2x80x9d. The process was repeated at 3 non-collinear locations. A best-fit plane (exact in the case of 3 points) was used to coordinate the movement of the device that was mounted to the Z-stage.
Prior art laser-based, dynamic focus techniques and/or associated xe2x80x9cdepth from focusxe2x80x9d are widely used over a range of scales and at various operating speeds. Exemplary systems operating at a microscopic scale are disclosed in U.S. Pat. Nos. 5,690,785, 4,710,908, 5,783,814, and 5,594,235, and selected pages of Chapter 7 entitled xe2x80x9cOptics for Data Storagexe2x80x9d in the book xe2x80x9cLaser Beam Scanningxe2x80x9d by Marcel Dekker, Inc., 1985. A desirable improvement in the memory processing or the processing of other microstructures would provide capability to generate and apply industry-leading small spot sizes to the applications with improved throughput. In turn, an improved figure of merit for resolution and speed in the presence of local depth variations which substantially exceed the DOF associated with the small spot sizes.
An object of the present invention is to provide a high-speed method and system for precisely positioning a waist of a material-processing laser beam to process microstructures within a laser-processing site.
It is an object of the invention to provide a method and system for high-speed laser processing of microstructures on a surface having three-dimensional coordinates wherein the surface has substantial local warpage, wedge, or other variations in depth. The variations introduce a requirement for high speed, 3-dimensional relative motion of the target and laser beam, within a die site for example, so as to dynamically and accurately position the beam waist. The beam waist, which may be less than 1 um in depth, is to substantially coincide with the 3D location of the microstructure.
It is an object of the invention to provide an improved xe2x80x9cspeed-resolution productxe2x80x9d in 3 dimensions-where the control system for the wafer movement preferably provides movement in 2 directions, and the high precision lens actuator provides beam focusing (e.g.: positioning of the beam waist) action in the third dimension.
It is an object of the invention to reduce the alignment time of the process by estimating a surface which is used to define a trajectory thereby eliminating or minimizing any requirement for die-by-die alignment or die-by-die focus measurements.
It is an object of the invention to maintain the correct focus height (i.e.: beam waist position) over the entire site (i.e.: several dice of a wafer), thereby improving over prior art focus solutions capable of only maintaining focus at locations adjacent to the focus location.
It is an object of the invention is to process dice on a wafer with a single xe2x80x9csite.xe2x80x9d The field size may allow as many as six or eight 64M DRAM dice to be processed at one time. This process is called multi-die align (MDA). The use of MDA affords a significant throughput improvement by reducing the number of alignment operations required to process the wafer from 1-per die to 1-per site. The prior art alignment operations may require roughly the same amount of time to perform as link cutting for a single die.
In carrying out the above objects and other objects of the present invention, a method for precisely positioning a waist of a material-processing laser beam to dynamically compensate for local variations in height of microstructures located on a plurality of objects spaced apart within a laser-processing site is provided. The method includes providing reference data which represents 3-D locations of microstructures to be processed within the site, positioning the waist of the laser beam along an optical axis based on the reference data, and positioning the objects in a plane based on the reference data so that the waist of the laser beam substantially coincides with the 3-D locations of the microstructures within the site.
The objects may be semiconductor dice of a semiconductor wafer wherein the microstructures are conductive metal lines of the dice.
The objects may be semiconductor memory devices.
The step of providing may include the step of measuring height of the semiconductor wafer at a plurality of locations about the site to obtain reference height data. The step of providing may further include the steps of computing a reference surface based on the reference height data and generating trajectories for the wafer and the waist of the laser beam based on the reference surface.
The reference surface may be planar or non-planar.
The method may further include varying size of the waist of the laser beam about the optical axis.
The step of providing may include the steps of reducing power of the material-processing laser beam to obtain a probe laser beam and utilizing the probe laser beam to perform the step of measuring.
Further in carrying out the above objects and other objects of the present invention, a system for precisely positioning a waist of a material-processing laser beam to dynamically compensate for local variations in height of microstructures located on a plurality of objects spaced apart within a laser-processing site is provided. The system includes a focusing lens subsystem for focusing a laser beam along an optical axis, a first actuator for moving the objects in a plane, and a second actuator for moving the focusing lens subsystem along the optical axis. The system further includes a first controller for controlling the first actuator based on reference data which represents 3-D locations of microstructures to be processed within the site, and a second controller for controlling of the second actuator also based on the reference data. The first and second actuators controllably move the objects and the focusing lens subsystem, respectively, to precisely position the waist of the laser beam and the objects so that the waist substantially coincides with the 3-D locations of the microstructures within the site.
A support supports the second actuator and the focusing lens subsystem for movement along the optical axis.
The system may further include a spot size lens subsystem for controlling size of the waist of the laser beam, a third actuator for moving the spot size lens subsystem wherein the support also supports the spot size lens subsystem and the third actuator for movement along the optical axis, and a third controller for controlling the third actuator.
The first actuator may be an x-y stage.
The second and third actuators may be air bearing sleds for supporting the focusing lens subsystem and the spot size lens subsystem, respectively, both mounted for sliding movement on the support.
A voice coil is coupled to its respective controller for positioning its air bearing sled along the optical axis.
The system may further include a position sensor such as a capacitive feedback sensor for sensing position of the focusing lens subsystem and providing a position feedback signal to the second controller.
The laser beam may be a Gaussian laser beam.
The system may further include a trajectory planner coupled to the first and second controllers for generating trajectories for the wafer and the waist of the laser beam. At least one of the trajectories may have an acceleration/deceleration profile.
The system may further include a modulator for reducing power of the material-processing laser beam to obtain a probe laser beam to measure height of the semiconductor wafer at a plurality of locations about the site to obtain reference height data. The system may include a computer for computing a reference surface based on the reference height data wherein the trajectory planner generates the trajectories based on the reference surface which may be planar or non-planar.
The invention improves upon the prior art by including two steps:
1. Height measurements are performed at multiple (typically 4 or more) points surrounding the die site.
2. The focus (beam waist) height is adjusted as the laser beam is positioned within the site so as to maintain best focus throughout the site based on fitting a surface to multiple height measurements.
A method for high speed laser processing of micro-structures having three dimensional coordinates includes the steps of:
Selecting a plurality of reference locations on a surface from which height data is to be obtained, obtaining height coordinates at the plurality of reference locations separate from but in proximity to micro-structures, estimating three dimensional locations of micro-structures from the coordinates of the reference locations, generating a trajectory adapted to position micro-structures relative to a location defining a laser processing beam axis, determining the position of an optical component disposed in path of the laser processing beam such that the corresponding position of the beam waist of the focused laser processing beam will substantially coincide with a coordinate of a micro-structure when the micro-structure is positioned to intersect the active laser processing beam, inducing relative movement between micro-structures and the location of a laser processing beam while coordinating the movement of the optical element in the path of the laser processing beam to dynamically adjust the position of the beam waist of the processing laser beam whereby the location of the beam waist substantially coincides with a coordinate of the micro-structure when it intersects the laser processing beam, providing a laser processing beam pulse to process the microstructure while relative movement is occurring between the micro-structures and the laser processing beam.
The height information will preferably be obtained from the same laser and optical path used for processing, but with reduced power (with a modulator used to reduce the power and avoid damage to the surface).
Alternatively, a separate tool may be used to measure the height of the surface at reference locations.
In a construction of the invention, the estimated surface location may be computed from a planar fit, higher order surface fit, through bilinear interpolation.
A straight line approximation may be used for micro-structures located in a row.
The preferred optical system has capability for both spot size selection and focus control.
The optical focusing system is preferably mounted on an air bearing sled.
In a preferred system, spot size adjustment is provided with zoom elements mounted on an air bearing sled which independently adjusts spot size.
In a preferred system a high precision voice coil motor is mounted to the optical box and operatively connected to the air bearing sled.
In a preferred system, the position of the focusing optical system is monitored with a high band width position sensor, such as a capacitive feedback sensor.
In a preferred construction the positioning of the lens or optical element provides Z-axis resolution of about 0.1 um with a half power bandwidth of about 150 Hz.
In a preferred construction of the present invention, the maximum velocity of the wafer movement stage during processing is in the range of about 50-150 mm/sec.
The preferred range of movement of the optical element corresponds to about 3 mm movement range of the beam waist along the Z direction.
In a construction of the invention, the response of the actuator controlling the beam waist position can correspond to an incremental change in depth within a duration of about 0.03 msec.
A numerical offset may be introduced to compensate for the thickness of overlying passivation layers covering the micro-structure, or other offsets with respect to the reference surface.
The spot size at the three-dimensional coordinate of the microstructure is preferably within 10% of the diffraction limited (smallest) spot size after relative movement of an optical element.
The energy enclosure at the three-dimensional coordinate of the microstructure preferably exceeds 95% size after relative movement of an optical element.
The peak energy of the processing laser spot will preferably exceed 90% of the maximum peak energy.
The laser beam may be substantially Gaussian and TEM00.
The z coordinate of the beam waist is preferably dynamically adjusted and follows a computed surface, such as a plane. The corresponding change in depth between any two structures, including adjacent structures in a row of microstructures, may exceed the Z-axis resolution of the optical system positioner within a die.
The z coordinate of the beam waist is preferably dynamically adjusted and follows a computed surface, such as a plane. The corresponding change in depth between any two structures, including adjacent die on the wafer, may exceed the DOF of the laser beam.
A dimension of a microstructure may be less than the wavelength of the laser, for example: 0.8 xcexcm width, 6 xcexcm length, 1 xcexcm thickness spaced apart by about 1.5 xcexcm-3 xcexcm from center-to-center.
The tolerable DOF of the laser beam may be on the order of or less than 1 wavelength of the laser processing beam.
The tolerable DOF of the laser beam may be less than 1 um.
The optical element may be moving the position of the beam waist in response to a continuous motion signal while the laser processing of the microstructure is occurring.
The optical element may be moving the position of the beam waist during the relative motion of the laser and micro-structures.
The relative motion of the lens may be constant, or may have acceleration/deceleration profiles provided by a trajectory planner.
A system of the present invention is able to operate with smaller spot sizes (which require better focus control) and thereby process devices with smaller geometry than prior memory repair systems due in part to superior focus control.
Dynamic Focus allows a system of the present invention to adapt to the non-parallel and non-planar topology that is typically found on real wafers and maintain acceptable focus over the full extent of a die site.
The method and system of the present invention is to be advantageously applied to semiconductor memory repair. However, it will be apparent that the present invention is also advantageous for microscopic laser processing applications where the depth of focus is small compared to the local height variations in the surface, and where the laser processing is to occur at high speed.